We have determined a crude structure of the apo form of bovine β-lactoglobulin, a protein of 162 amino acid residues with a molecular mass of 18 kDa, at a low pH on the basis of data collected using only homonuclear ¹H NMR spectroscopy. An ensemble of protein conformations was calculated with the distance-geometry algorithm for NMR applications (DYANA). The monomeric protein at low pH adopts a β-barrel fold, well-superimposable on the structure determined by X-ray crystallography for the dimer at physiological pH. NMR evidence suggests the presence of disordered loop regions and terminal segments. Structural differences between the monomer at pH 2 and the dimer at pH 7, obtained by X-ray crystallography, are discussed, paying particular attention to surface electrostatic properties, in view of the high charge state of the protein at low pH.